This invention relates to pressure filters for separating liquids and particulate solids. More particularly, the invention is concerned with tube pressure filters of the kind which comprise (a) a pair of generally coaxial tubular bodies arranged one within the other and adapted to be supported in a generally upright position, (b) an impermeable elastic sleeve disposed within and secured to the outer tubular body, (c) a filter element disposed around and supported by the inner tubular body, (d) outlet means for the discharge of filtrate which has passed through the filter element and (e) means for displacing the tubular bodies axially relative to one another between first and second positions, the arrangement being such that in the first position of said tubular bodies they co-operate with each other to define a closed annular chamber which is divided into generally coaxial and non-intercommunicating inner and outer compartments by said impermeable elastic sleeve, the inner compartment having an inlet for feed material comprising a mixture of liquid and particulate solid material and the outer compartment having an inlet for a hydraulic fluid under pressure, and in the second position of said tubular bodies said annular chamber is open to enable particulate solid material to be discharged from the inner compartment. Hereinafter a tube pressure filter of this kind will be referred to as "a tube pressure filter of the kind set forth". Further details of the construction and operation of tube pressure filters of the kind set forth are given in, for example, British Patent Specification Nos. 907,485; 1,240,465; 1,240,466; 1,317,887; 1,340,858; 1,351,142; 1,351,943; 1,355,334; 1,336,856; 1,362,655; and 1,386,256.
When a tube pressure filter of the kind set forth is to be used, there is usually established a filtration cycle in which the tubular bodies are moved to their first position and feed material to be pressure filtered (in the form of a slurry comprising a mixture of liquid and particulate solid) is fed into the inner compartment through the inlet of the latter, and hydraulic fluid is introduced into the outer compartment through the inlet of the latter. The pressure exerted by the hydraulic fluid is increased and this causes filtrate, i.e. liquid, to be expressed through pores in the filter element, and a filter cake of the particulate solid material to be built up on the outer surface of the filter element. The filtrate is discharged from the tube pressure filter through the outlet means provided therefor. The hydraulic fluid is then withdrawn from the outer compartment, and the tubular bodies are moved to their second position whereupon the filter cake can be discharged from the inner compartment. The tubular bodies are then returned to their first position and the filtration cycle can be repeated.
There have been described various arrangements whereby the feed material to be pressure filtered can be fed into the inner compartment of a tube pressure filter of the kind set forth. In one arrangement, the inlet of the inner compartment comprises a plurality of apertures distributed around the lower end of the inner tubular body whereby feed material can be fed into the inner compartment uniformly around the bottom thereof. Advantageously, these apertures communicate with an antechamber into which the feed material to be pressure filtered can be introduced under pressure through a feed conduit. The apertures are in the form of slots and are normally closed by a ring of elastomeric material which is conveniently of rectangular or L-shaped cross-section and which is disposed around the outside of the inner tubular body and acts as a non-return valve, the feed material being forced past the ring of elastomeric material during the introduction of the feed material into the inner compartment and being prevented by the ring of elastomeric material from returning to the ante-chamber during the pressure filtration. It has been found from practical experience that problems may arise with the arrangement described above; firstly during discharge of the filter cake there is a tendency for the feed material which is to be pressure filtered to leak around the ring of elastomeric material; and secondly, during pressure filtration the ring of elastomeric material tends to be drawn or forced into the apertures forming the inlet to the inner compartment with consequent risk of damage to and failure of the elastomeric ring.